1. Field of the Invention
This invention relates to the field of control of oxygen enhanced boilers. A method for controlling emissions in a boiler delivering a controlled steam output is disclosed. One or more oxygen enriched oxidant streams are introduced into the combustion chamber. The flue gas characteristics, as well as other boiler performance variables, are monitored. At least one controller actuates on the oxidant injections to control combustion. The controller calculates the optimum oxidant distribution in real-time for a series of injection points to minimize undesired flue gas emissions while maintaining the desired steam conditions and boiler operating conditions. Due to the multivariable nature of the boiler and its complexity, the controller can be a multivariable advanced controller. The controller can be adapted as a retrofit, by integration with existing control technology, or it can be installed as an integral unit in a new application. A real time optimizer is continuously calculating the optimum set points for the controller.
2. Related Art
In a combustion process for steam generation, a fuel-fired boiler delivers steam at constant temperature and pressure and at user-defined demands. In order to achieve these requirements, the boiler consists of a variety of control (output) variables and manipulating (input) variables. The most common control (output) variables are the steam mass flow rate, the water level in the drum, the steam pressure in the drum, the boiler pressure and the gas composition of the flue gases. Meanwhile, the typical manipulating (input) variables are the fuel flow rate, the oxidant flow rate, the feed water flow rate and the flue gas fan. As can be seen, the control and manipulating variables are the same or similar regardless of the fuel type; coal-fired boilers represent just an example of the technology.
A typical boiler control system has a burner subassembly and a steam subassembly. In the boiler, fuel is mixed with an oxidant to burn and generate heat to be transferred to feed water to create steam. The amount of steam required is quickly varied by an actuator valve; however, replenishment of the steam is typically more slowly accomplished by varying the combustion rate. Oxygen in the flue gas is measured so that a pre-selected range of excess oxygen in the flue gas is maintained; the oxidant input is varied to ensure good combustion and provide the amount of oxygen necessary to achieve the pre-selected range of excess oxygen in the flue gas. Adequate pressure in the boiler is maintained, and the pressure in the water drum ensures the quality of steam delivered.
Thus, the typical approach to boiler control employs dedicated controllers for each subassembly. Often, steam subassembly control is provided by controlling the steam flow rate using a steam valve. Pressure in the drum is controlled with the fuel flow rate and compensated by the steam flow rate. The water level in the drum is controlled by the feed water valve and also compensated by the steam flow rate. The pressure in the boiler is controlled by the draft created by the flue gas fan. Simultaneously, burner control is provided by monitoring the oxygen excess in the flue gases and manipulating the flow rate of the oxidant, thereby controlling the stoichiometry of the combustion.
In U.S. Pat. No. 5,688,296, a control system for an integrated gasification combined cycle steam system (IGCC) locally in the gasifier and globally in the cycle is disclosed. Information from on-line gas analyzers is not utilized in the control. In U.S. patent application Nos. 2002/0192609 A1 and U.S. application No. 2003/0000436 A1, real-time flue gas concentrations are monitored and the data used to adjust the air-to-fuel ratio and increase combustion efficiency. This is extended to use a model based predictive control to control the flue gas concentrations while using the smallest amount of air while including secondary and tertiary air supplies. U.S. patent application No. 2003/0014131 A1 uses the inputs and outputs of a boiler to create a steady state model. The values are then optimized to find the optimum input to the plant. The static model is converted into a dynamic model and then the multivariable problem is executed.
In U.S. Pat. No. 6,507,774, a steady-state neural network model and a nonlinear optimization are disclosed. Steady state fuel conditions are controlled to optimize operating conditions, but the system cannot accommodate dynamic operating conditions. In U.S. Pat. No. 6,093,372, control of oxygen supply in a gasifier based on on-line sensors for gas products is disclosed.
However, nothing in the art discloses a controller for optimizing oxidant distribution in real-time for a series of injection points to minimize undesired flue gas emissions while maintaining the desired steam conditions and boiler operating conditions. Thus, process control by oxygen enrichment in a variety of injection points in boilers to minimize the pollutant concentrations in the flue gas while simultaneously maintaining the required steam properties and the desired load and continuously updating the conditions that make the boiler operate at the maximum efficiency is not disclosed. The present application discloses applying oxygen-enriched inputs in boilers that use on-line flue gas measurements and an advanced multivariable controller supplying oxygen rich inputs in a plurality of inputs.
Thus, a problem associated with boilers that precede the present invention is that they do not provide oxygen enrichment of the combustion process while at the same time providing adjustment of the steam generation process to accommodate changing characteristics of the boiler.
Yet another problem associated with boilers that precede the present invention is that they do not provide control of the combustion process to reduce NOx and other undesired combustion by-products while at the same time providing optimal steam generation control.
Still another problem associated with boilers that precede the present invention is that they do not provide integration of combustion control with steam generation control to accommodate varying conditions in the combustion process and their consequent effects on the characteristics of the boiler operation.
Another problem associated with boilers that precede the present invention is that they cannot be readily adapted to retrofit existing boilers with oxygen enhancement for the combustion and simultaneously provided with integrated control of the combustion process and the steam generation process.
For the foregoing reasons, there has been defined a long felt and unsolved need for a boiler that overcomes the aforementioned problems and provides an economical, reliable boiler control system that can be either retrofitted to existing equipment or designed into new construction.